In recent years, oil and natural gas produced are more likely to contain large amounts of wet carbon dioxide and hydrogen sulfide gas, and martensitic stainless steels such as 13 Cr stainless steels have been used for well drilling and transportation of oil and natural gas instead of conventional carbon steels. However, conventional martensitic stainless steels are excellent in corrosion resistance against wet carbon dioxide gas (hereinafter, refer to as “corrosion resistance”), but insufficient in stress corrosion cracking resistance against wet hydrogen sulfide (hereinafter, refer to as “stress corrosion cracking resistance”), and martensitic stainless steels with improved stress corrosion cracking resistance while maintaining excellence in strength, toughness, and corrosion resistance have been desired.
Martensitic stainless steels meeting the requirements of stress corrosion cracking resistance in addition to strength, toughness, and corrosion resistance were disclosed in Patent Documents 1 to 3.
On the one hand, martensitic stainless steels with improved stress corrosion cracking resistance under environments in which a partial pressure of hydrogen sulfide exceeds 0.01 bar were also proposed, and disclosed, for example, in Patent Documents 4 and 5.
Further, high strength martensitic stainless steels having excellent corrosion resistance was disclosed in Patent Document 6, which was already granted as a patent.
However, martensitic stainless steels disclosed in Patent Documents 1 to 3 have excellent stress corrosion cracking resistance in environments involving an extremely small amount of hydrogen sulfide, but there is a problem that martensitic stainless steels cannot be used in environments involving large amounts of hydrogen sulfide, since stress corrosion cracking occurs in environments in which a partial pressure of hydrogen sulfide exceeds 0.01 bar.
Also, martensitic stainless steels according to Patent Documents 4 and 5 cannot fully prevent stress corrosion cracking due to hydrogen sulfide.
Further, in any one of the abovementioned martensitic stainless steels, there are problems that from the viewpoint of strength, attempts to strengthen the steel result in significant deterioration of toughness and stress corrosion cracking resistance, whereby there is no other choice but to sacrifice either one of strength or toughness and stress corrosion cracking resistance. Therefore, there is a drawback that the martensitic stainless steel cannot be applied, for example, to deep oil and gas wells where high strength, excellent stress corrosion cracking resistance, excellent corrosion resistance, and good toughness are simultaneously required.
To solve the problems in conventional technology, Patent Document 6 disclosed a low C-high Cr stainless steel tube having minimum yield strength of 862 MPa which can be used in environments involving large amounts of hydrogen sulfide without causing stress corrosion cracking while maintaining excellent corrosion resistance by simultaneously improving strength, stress corrosion cracking resistance, and toughness of conventional martensitic stainless steels, and a manufacturing method thereof.